1. Field of the Invention
The present invention relates to a controller of an electric power-assist steering system that provides steering assist by using assist torque with a motor, more specifically to a controller of an electric power-assist steering system that utilizes hysteresis of assist torque with a motor, so as to better steering feeling.
2. Description of the Related Art
An electric power-assist steering system is a steering assist system that generates assist torque with a motor in accordance to magnitude of steering torque of a vehicle, and transmits this assist torque to a steering system, thereby to reduce steering force of a driver when the driver steers a steering wheel. In such an electric power-assist steering system, a drive current command value for a motor is calculated by subtracting a torque current correction value defined depending on the steering torque of the steering wheel from a base current command value. In this manner, since the motor provides a steering wheel with an appropriate assist torque having hysteresis depending on the steering speed, a preferable steering feeling can be realized (see JP 3176899 B, for example). However, this scheme defines a torque current correction value depending on the steering speed of the steering wheel, and the steering speed thereof may become zero when the steering wheel is maintained at a constant steering angle while the vehicle runs in a curve section, for example. In this case, the torque current correction value becomes zero, so that the hysteresis width of the assist torque with the motor becomes decreased, resulting in deterioration of steering feeling.
To address this problem, there has been disclosed a technique to perform a torque current correction even when the steering speed becomes zero, so as to increase the hysteresis width of an assist torque with a motor to better steering feeling (see JP 4034294 B, for example). This technique determines whether the status of the steering wheel is “turn” or “return” based on a relation between the orientation of the steering torque (sign) and the orientation of the rotational rate of the motor (sign). Specifically, it is determined that the status of the steering wheel is “turn” if the orientation of the steering torque (sign: + or −) agrees with the orientation of the rotational speed of the motor (sign: + or −), or and it is determined that the status of the steering wheel is “return” if the orientation of the steering torque (sign: + or −) disagrees with the orientation of rotational speed of the motor (sign: + or −). In other words, it is determined that the steering status is “turn” when the motor torque (the assist torque with the motor) acts as a steering assist, and it is determined that the steering status is “return” when the motor torque applies an action reversal to the steering assist action. And the torque current correction value is switched depending on the steering status of either “turn” or “return”. At this time, when switching the steering status between “turn” and “return”, the torque current correction value is defined to be zero if the steering status is “turn”, and is defined not to be zero if the steering status is “return”. The above control increases the hysteresis width of the assist torque with the motor, thereby to better steering feeling.
However, in such a technique of JP 4034294 B, when the status of the steering wheel is switched between “turn” and “return”, the torque current correction value drastically changes between a value of zero (0) and a value other than zero (±α), thus the drive current command value of the motor also drastically changes in a discontinuous way. Consequently, the steering torque significantly changes when the status of the steering wheel is switched between “turn” and “return”, resulting in deterioration of steering feeling.
To address the above problem, the present invention has an object to provide a controller of an electric power-assist steering system in which the hysteresis is enhanced to prevent a drastic change of a drive current command value for a motor even at the time of switching the status of the steering wheel between “turn” and “return”, realizing preferable steering feeling without a great variation of steering torque.